Unmanned or uninhabited aerial vehicles (“UAVs”), commonly referred to as drones, are aircraft without a human pilot aboard. UAVs may be remotely piloted and/or can navigate autonomously. A variety of propulsion technologies are currently utilized. UAVs that are in use today vary in size from Micro Air Vehicles (“MAVs”) having no dimension larger than 15 cm to UAVs with wingspans of several tens of feet. Depending upon the application, UAVs can carry cameras, sensors, communications equipment, and/or other payloads.
For operation in highly-congested, highly cluttered environments like urbanized areas, whether indoors or outdoors, UAVs are typically required to be able to have high maneuverability at low speeds, and the capability of hovering. UAVs that have these capabilities are typically regarded to fall within three classes: rotating-wing configurations, like helicopters and tilt-rotors, flapping-wing configurations (emulating birds or insects), and fixed wing configurations with powered-lift capability. For any aircraft, low-speed flight and hovering flight are inherently power-hungry and rotary-wing aircraft tend to exhibit the highest efficiency in hover and low-speed flight relative to other propulsion systems.
The design of a propulsion system for a smaller UAV can be significantly different from the design of a propulsion system for a larger piloted aircraft. Large aircraft, such as commercial airliners and helicopters, operate at Reynolds numbers in the tens of millions, whereas smaller UAVs such as MAVs can operate in a Reynolds number regime of approximately 10,000 to 50,000. The primary implication of operation at comparatively lower Reynolds numbers is a reduction in the maximum lift capacity of an airfoil and increases in pressure drag and skin friction drag when the flow remains attached to the airfoil. Together, these effects can result in extremely low lift-to-drag ratios for airfoils in low Reynolds number flows. The degraded performance of airfoils is an obstacle faced by both fixed and rotary wing MAVs, but is especially critical for the latter, as they spend a large portion of flight in power-intensive hovering and low-speed conditions.
A variety of propulsion systems are currently being utilized in commercial UAVs designed for hovering including coaxial multirotor propulsion systems, radial multirotor propulsion systems, and ducted fans. Coaxial multirotor systems utilize a pair of rotors that are aligned coaxially and configured to rotate in opposite directions. In order to control pitch, yaw, and roll, a coaxial multirotor system typically includes a mechanism that adjusts the pitch of the propeller blades. Usually the pitch change is achieved by mounting servos with mechanical linkages to the propeller blades so that the angle of the propellers can be adjusted. Radial multirotor systems can overcome some of the complexity of coaxial multirotor systems by eliminating the need to tilt the propeller blades to control pitch, yaw, and roll.
Radial multirotor systems are utilized by a class of UAVs that includes quadcopters, hexcopters, and octocopters. Radial multirotor systems typically utilize at least two pairs of fixed pitch propellers. Typically, the pairs of rotors do not all share the same direction of rotation and variations in the angular velocity of the rotors can be utilized to control lift and torque. The principles utilized in the construction of radial multirotor systems can also be utilized to construct UAVs that include coaxial pairs of radial rotors.
UAVs including coaxial and/or radial multirotor propulsion systems often include frames that enclose the rotors to protect the rotors and/or environment during flight. A distinction can be drawn between the use of a frame to protect a propeller and a ducted fan propulsion system. A ducted fan is a propulsion arrangement whereby a mechanical fan, which is a type of propeller, is mounted within a shroud or duct. The duct reduces losses in thrust from the tips of the props, and varying the cross-section of the duct can advantageously affect velocity and pressure of airflow.